The overall objectives of these studies are to develop and evaluate techniques for obtaining quantitative measurements of the permeability characteristics of human and canine dentine in vitro. Using dogs, these studies will also be repeated in vivo and compared to the data obtained in vitro. Some of the variables that will be evaluated include, 1) the quantitative contribution of the walls of occlusal cavities versus the pulpal floor to overall cavity permeability, 2) the influence of dentine thickness, 3) the influence of dentine surface area, and the effects that molecular size and charge have on the rate of dentine permeation by various substances. Attempts will be made to increase and decrease the permeability of dentine and to express these changes in quantitative terms. The spectrum of agents to be used in evaluating dentine permeability will include agents common to the oral fluids, bacterial products and drugs used in dentistry. If the objectives of this study are fulfilled, the data obtained should permit estimations of permeability coefficients of substances if the thickness and surface area of dentine are known or, using substances of known permeability coefficients, the "effective" volume of dentine that is being permeated by that substance.